Evacuated gratings and methods of manufacturing

ABSTRACT

Improvements to gratings for use in waveguides and methods of producing them are described herein. Deep surface relief gratings (SRGs) may offer many advantages over conventional SRGs and Bragg gratings, an important one being a higher S-diffraction efficiency. In one embodiment, deep SRGs can be implemented as polymer surface relief gratings or evacuated Bragg gratings (EBGs). EBGs can be formed by first recording a holographic polymer dispersed liquid crystal (HPDLC) grating. Removing the liquid crystal from the cured grating provides a polymer surface relief grating. Polymer surface relief gratings have many applications including for use in waveguide-based displays.

CROSS-REFERENCED APPLICATIONS

This application claims priority to U.S. Provisional Application62/893,715 filed on Aug. 29, 2019, the disclosure of which is includedherein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present invention generally relates to waveguides and methods forfabricating waveguides and more specifically to waveguide displayscontaining gratings formed in a multi-component mixture from which onematerial component type is removed and methods for fabricating saidgratings.

BACKGROUND

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One subclass includes optical waveguides,which are structures that can guide electromagnetic waves, typicallythose in the visible spectrum. Waveguide structures can be designed tocontrol the propagation path of waves using a number of differentmechanisms. For example, planar waveguides can be designed to utilizediffraction gratings to diffract and couple incident light into thewaveguide structure such that the in-coupled light can proceed to travelwithin the planar structure via total internal reflection (TIR).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within or on thesurface of the waveguides. One class of such material includes polymerdispersed liquid crystal (PDLC) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(HPDLC) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize, and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal (LC) micro-droplets, interspersed withregions of clear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays forAugmented Reality (AR) and Virtual Reality (VR), compact Heads UpDisplays (HUDs) for aviation and road transport, and sensors forbiometric and laser radar (LIDAR) applications. As many of theseapplications are directed at consumer products, there is a growingrequirement for efficient low cost means for manufacturing holographicwaveguides in large volumes.

SUMMARY OF THE DISCLOSURE

Many embodiments are directed to polymer grating structures, theirdesign, methods of manufacture, and materials.

Various embodiments are directed to a waveguide based device including:

-   -   a waveguide supporting a polymer grating structure for        diffracting light propagating in total internal reflection in        said waveguide,    -   wherein the polymer grating structure comprises:        -   a polymer network; and        -   air gaps between adjacent portions of the polymer network.

In still various other embodiments, the polymer grating structure mayfurther include an isotropic material between adjacent portions of thepolymer network, where the isotropic material has a refractive indexhigher or lower than the refractive index of the polymer network.

In still various other embodiments, the isotropic material may occupy aspace at a bottom portion of the space between adjacent portions of thepolymer network and the air may occupy the space from above the topsurface of the isotropic material to the modulation depth.

In still various other embodiments, the isotropic material may include abirefringent crystal material.

In still various other embodiments, the birefringent crystal materialmay include a liquid crystal material.

In still various other embodiments, the birefringent crystal materialmay be a material of higher refractive index than the polymer.

In still various other embodiments, the polymer grating structure mayhave a modulation depth greater than a wavelength of visible light.

In still various other embodiments, the polymer grating structure mayinclude a modulation depth and a grating pitch, where the modulationdepth is greater than the grating pitch.

In still various other embodiments, the waveguide may include twosubstrates and the polymer grating structure may be either sandwichedbetween the two substrates or positioned on an external surface ofeither substrate.

In still various other embodiments, the Bragg fringe spacing of thepolymer network may be 0.35 μm to 0.8 μm and the grating depth of thepolymer network may be 1 μm to 3 μm.

In still various other embodiments, the ratio of grating depth of thepolymer network to the Bragg fringe spacing may be 1:1 to 5:1.

In still various other embodiments, the waveguide display may furtherinclude a picture generating unit, where the polymer grating structuremay include a waveguide diffraction grating.

In still various other embodiments, the waveguide diffraction gratingmay be configured as a multiplexing grating.

In still various other embodiments, the waveguide diffraction gratingmay be configured to accept light from the picture generating unit whichincludes multiple images.

In still various other embodiments, the waveguide diffraction gratingmay be configured to outcouple light from the waveguide.

In still various other embodiments, the waveguide diffraction gratingmay be configured as a beam expander.

In still various other embodiments, the waveguide diffraction gratingmay be configured to incouple light including image data generated fromthe picture generating unit.

In still various other embodiments, the waveguide diffraction gratingmay further be configured to incouple S-polarized light with a highdegree of efficiency.

In still various other embodiments, the diffraction grating may befurther configured to incouple S-polarized light at an efficiency of 70%to 95% at a Bragg angle.

In still various other embodiments, the diffraction grating may befurther configured to incouple P-polarized light at an efficiency of 25%to 50% at a Bragg angle.

In still various other embodiments, the refractive index differencebetween the polymer network and the air gaps may be 0.25 to 0.4.

In still various other embodiments, the refractive index differencebetween the polymer network and the birefringent crystal material may be0.05 to 0.2.

In still various other embodiments, the polymer grating structure mayinclude a two-dimensional lattice structure or a three-dimensionallattice structure.

In still various other embodiments, the waveguide display may furtherinclude another grating structure.

In still various other embodiments, the polymer grating structure mayinclude an incoupling grating and the other grating structure comprisesa beam expander or an outcoupling grating.

Further, various embodiments are directed to a waveguide displayincluding:

-   -   a waveguide supporting a polymer grating structure for        diffracting light propagating in total internal reflection in        said waveguide,        -   where the polymer grating structure include:            -   a polymer network; and            -   a birefringent crystal material between adjacent                portions of the polymer network, where the birefringent                crystal material has a higher refractive index than the                polymer.

Further, various embodiments are directed to a method for fabricating adeep surface relief grating (SRG), the method includes:

-   -   providing a mixture of monomer and liquid crystal;    -   providing a substrate;    -   coating a layer of the mixture on a surface of the substrate;    -   applying holographic recording beams to the layer to form a        holographic polymer dispersed liquid crystal grating comprising        alternating polymer rich regions and liquid crystal rich        regions; and    -   removing at least a portion of the liquid crystal in the liquid        crystal rich regions to form a polymer surface relief grating.

In still various other embodiments, the monomer comprises acrylates,methacrylates, vinyls, isocynates, thiols, isocyanate-acrylate, and/orthioline.

In still various other embodiments, the mixture may further include atleast one of a photoinitiator, a coinitiator, or additional additives.

In still various other embodiments, the thiols may includethiol-vinyl-acrylate.

In still various other embodiments, the photoinitiator may includephotosensitive components.

In still various other embodiments, the photosensitive components mayinclude dyes and/or radical generators.

In still various other embodiments, providing a mixture of monomer andliquid crystal may include:

-   -   mixing the monomer, liquid crystal, and at least one of a        photoinitiator, a coinitiator, multifunctional thiol, or        additional additives;    -   storing the mixture in a location absent of light at a        temperature of 22° C. or less;    -   adding additional monomer;    -   filtering the mixture through a filter of 0.6 μm or less; and    -   storing the filtered mixture in a location absent of light.

In still various other embodiments, the substrate may include a glasssubstrate or plastic substrate.

In still various other embodiments, the substrate may include atransparent substrate.

In still various other embodiments, the method may further includesandwiching the mixture between the substrate and another substrate withone or more spacers for maintaining internal dimensions.

In still various other embodiments, the method may further includeapplying a non-stick release layer on one surface of the othersubstrate.

In still various other embodiments, the non-stick release layer mayinclude a fluoropolymer.

In still various other embodiments, the method may further includerefilling the liquid crystal rich regions with a liquid crystalmaterial.

In still various other embodiments, the liquid crystal material may havea different molecular structure than the previously removed liquidcrystal.

In still various other embodiments, removing at least a portion of theliquid crystal may include removing substantially all of the liquidcrystal in the liquid crystal rich regions.

In still various other embodiments, removing at least a portion of theliquid crystal further may include leaving at least a portion of theliquid crystal in the polymer rich regions.

In still various other embodiments, the method may further includeapplying a protective layer over the deep SRG.

In still various other embodiments, the protective layer may include ananti-reflective layer.

In still various other embodiments, the protective layer may includesilicate or silicon nitride.

In still various other embodiments, applying a protective layer mayinclude depositing the protective layer on the deep SRG.

In still various other embodiments, depositing the protective layer mayinclude chemical vapor deposition.

In still various other embodiments, the chemical vapor deposition may bea nanocoating process.

In still various other embodiments, the protective layer may include aparylene coating.

In still various other embodiments, the liquid crystal rich regions mayinclude air gaps after removing at least a portion of the liquid crystalin the liquid crystal rich regions.

In still various other embodiments, the method may further includecreating a vacuum in the air gaps or filling the air gaps with an inertgas.

In still various other embodiments, removing at least a portion ofliquid crystal may include washing the holographic polymer dispersedliquid crystal grating with a solvent.

In still various other embodiments, washing the holographic polymerdispersed liquid crystal grating may include immersing the holographicpolymer dispersed liquid crystal grating in the solvent.

In still various other embodiments, the solvent may include isopropylalcohol.

In still various other embodiments, the solvent may be kept at atemperature lower than room temperature while washing the holographicpolymer dispersed liquid crystal grating.

In still various other embodiments, removing at least a portion of theliquid crystal may further include drying the holographic polymerdispersed liquid crystal grating with a high flow air source.

In still various other embodiments, the method may further includecuring the holographic polymer dispersed liquid crystal grating.

In still various other embodiments, curing the holographic polymerdispersed liquid crystal grating may include exposing the holographicpolymer dispersed liquid crystal grating to a low intensity white lightfor a period of about an hour.

In still various other embodiments, the polymer surface relief gratingmay be configured to incouple S-polarized light at an efficiency of 70%to 95%.

In still various other embodiments, the polymer surface relief gratingmay be further configured to incouple P-polarized light at an efficiencyof 25% to 50%.

In still various other embodiments, the refractive index differencebetween the polymer network and the air gaps may be 0.25 to 0.4.

In still various other embodiments, the refractive index differencebetween the polymer network and the liquid crystal material may be 0.05to 0.2.

In still various other embodiments, the polymer surface relief gratingmay include a Bragg fringe spacing of 0.35 μm to 0.8 μm and the gratingdepth of 1 μm to 3 μm.

In still various other embodiments, the polymer surface relief gratingmay include a ratio of Bragg fringe spacing to grating depth of 1:1 to5:1.

In still various other embodiments, the liquid crystal content in themixture of monomer and liquid crystal may be approximately 20% to 50%.

In still various other embodiments, the liquid crystal in the mixture ofmonomer and liquid crystal may include liquid crystal singles.

In still various other embodiments, the liquid crystal singles mayinclude cyanobiphenyl and/or pentylcynobiphenyl.

Further, various embodiments are directed to a method for fabricating adeep SRG, the method may include:

-   -   providing a mixture of monomer and a substance;    -   providing a substrate;    -   coating a layer of the mixture on a surface of the substrate;    -   applying holographic recording beams to the layer to form a        holographic polymer dispersed grating comprising alternating        polymer rich regions and substance rich regions; and    -   removing at least a portion of the substance in the substance        rich regions to form a polymer surface relief grating.

In still various other embodiments, the monomer may be reactive to theholographic recording beams and the substance may be unreactive to theholographic recording beams.

In still various other embodiments, the monomer and the substance may bea miscible mixture before the applying holographic recording beams andthe monomer and the substance become an immiscible mixture after theapplying holographic recording beams.

In still various other embodiments, the substance may include liquidcrystal.

In still various other embodiments, the substance may include a liquidcrystal single.

In still various other embodiments, the substance may include asolvents, non-reactive monomers, inorganics, and/or nanoparticles.

Further, various embodiments are directed to a waveguide display mayinclude:

-   -   an emissive array emitting light in a first wavelength band;    -   a collimation lens for projecting image modulated light from        said emissive array over a field of view; and    -   a waveguide supporting:        -   input and output SBGs with high diffraction efficiency for            S-polarized light in said first wavelength band; and        -   input and output SBGs with high diffraction efficiency for            P-polarized light in said first wavelength band.

In still various other embodiments, said waveguide may further supportSBGs for diffracting S-polarized and P-polarized light in a secondwavelength band emitted by said emissive array.

In still various other embodiments, said emissive array may be an OLEDarray.

In still various other embodiments, said waveguide may be curved in atleast one plane.

In still various other embodiments, said waveguide may be fabricatedfrom plastic.

In still various other embodiments, said emissive array may be spatiallydistorted to pre-compensate for wavefront distortion produced by curvedsurfaces in said waveguide.

In still various other embodiments, said emissive array may be formed ona curved or flexible substrate to pre-compensate for wavefrontdistortion produced by curved surfaces in said waveguide.

In still various other embodiments, at least one of said gratings may beone of a Bragg grating recorded in a photopolymer a Bragg gratingrecording in a liquid crystal and monomer mixture, a deep surface reliefgrating, a hybrid surface relief/Bragg grating.

In still various other embodiments, said waveguide may support eyeprescription optical surfaces.

In still various other embodiments, said emissive may have a pixel arraypatterned using multiplicities of elements including at least oneselected from the group of polygons of identical size, polygons ofidentical shape, polygons varying in size across the array, polygonsvarying in shape across the array, Penrose tiles and elements formingnon repeating patterns.

Further, various embodiments are directed to a method for forming animage using a waveguide, the method including:

-   -   providing an emissive array emitting light in a first wavelength        band, a collimation lens and a waveguide supporting input and        output gratings with high diffraction efficiency for S-polarized        light in said first wavelength band and supporting input and        output ratings with high diffraction efficiency for P-polarized        light in said first wavelength band;    -   collimating image light emitted by the emissive array using the        collimation lens;    -   coupling image modulated S-polarized light from said OLED array        into a total internal reflection path in the waveguide using the        S-diffracting input grating;    -   coupling image modulated P-polarized light from said OLED array        into a total internal reflection path in the waveguide using the        P-diffracting input grating;    -   beam expanding and extracting S-polarized light from the        waveguide for viewing; and    -   beam expanding and extracting P-polarized light from the        waveguide for viewing.

In still various other embodiments, said emissive array may be an OLEDarray.

In still various other embodiments, the method may further include thestep of providing a curved optical surface supported by said waveguide;predistorting the pixel pattern on said emissive array, formingpredistorted wavefronts using said collimation lens; reflecting saidpredistorted wavefront light at said curved optical surface; and forminga planar wavefront from said predistorted wavefront using the opticalpower of said curved optical surface.

In still various other embodiments, said curved optical surface may be aprescription optical surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention.

FIG. 1A conceptually illustrates a step of a method for fabricating asurface relief grating in which a mixture of monomer and liquid crystaldeposited on a transparent substrate is exposed to holographic exposurebeams in accordance with an embodiment of the invention.

FIG. 1B conceptually illustrates a step of a method for fabricating asurface relief grating from an HPDLC Bragg grating formed on atransparent substrate in accordance with an embodiment of the invention.

FIG. 1C conceptually illustrates a step of a method for fabricating asurface relief grating in which liquid crystal is removed from an HPDLCBragg grating to form a polymer surface relief grating in accordancewith an embodiment of the invention.

FIG. 1D conceptually illustrates a step of a method for covering asurface relief grating with a protective layer in accordance with anembodiment of the invention.

FIG. 2 is a flow chart conceptually illustrating a method for forming apolymer surface relief grating from an HPDLC Bragg grating formed on atransparent substrate in accordance with an embodiment of the invention.

FIG. 3 is an example implementation of a polymer surface relief gratingor evacuated Bragg grating.

FIG. 4A conceptually illustrates a step of a method for fabricating asurface relief grating in which a mixture of monomer and liquid crystaldeposited on a transparent substrate is exposed to holographic exposurebeams in accordance with an embodiment of the invention.

FIG. 4B conceptually illustrates a step of a method for fabricating asurface relief grating from an HPDLC Bragg grating formed on atransparent substrate in accordance with an embodiment of the invention.

FIG. 4C conceptually illustrates a step of a method for fabricating asurface relief grating in which liquid crystal is removed from an HPDLCBragg grating to form a polymer surface relief grating in accordancewith an embodiment of the invention.

FIG. 4D conceptually illustrates a step of a method for fabricating asurface relief grating in which the surface relief grating is partiallyrefilled with liquid crystal to form a hybrid surface relief-Bragggrating in accordance with an embodiment of the invention.

FIG. 4E conceptually illustrates a step of a method for fabricating asurface relief grating in which a hybrid surface relief-Bragg grating iscovered with a protective layer in accordance with an embodiment of theinvention.

FIG. 5 is a flow chart conceptually illustrating a method for forming ahybrid surface relief-Bragg grating in accordance with an embodiment ofthe invention.

FIG. 6 is a graph showing calculated P-polarized and S-polarizeddiffraction efficiency versus incidence angle for a 1-micrometerthickness deep surface relief grating in accordance with an embodimentof the invention.

FIG. 7 is a graph showing calculated P-polarized and S-polarizeddiffraction efficiency versus incidence angle for a 2-micrometerthickness deep surface relief grating in accordance with an embodimentof the invention.

FIG. 8 is a graph showing calculated P-polarized and S-polarizeddiffraction efficiency versus incidence angle for a 3-micrometerthickness deep surface relief grating in accordance with an embodimentof the invention.

FIGS. 9A and 9B illustrate scanning electron microscope images ofmultiple embodiments including different thiol concentrations.

FIGS. 10A and 10B are images comparing an HPDLC Bragg grating and apolymer surface relief grating or evacuated Bragg grating.

FIGS. 11A and 11B are two plots comparing an HPDLC Bragg grating and apolymer surface relief grating or evacuated Bragg grating.

FIGS. 12A and 12B are two plots of S-diffraction efficiency andP-diffraction efficiency of two example polymer surface relief gratingswith different depths.

FIGS. 13A and 13B are two different plots of S-diffraction efficiencyand P-diffraction efficiency of various example polymer surface reliefgratings produced with different initial liquid crystal concentrations.

FIGS. 14A and 14B are two different plots of S-diffraction efficiencyand P-diffraction efficiency of various example polymer surface reliefgratings produced with different initial liquid crystal concentrations.

FIG. 15 conceptually illustrates a waveguide display in accordance withan embodiment of the invention.

FIG. 16 conceptually illustrates a waveguide display having twoair-spaced waveguide layers in accordance with an embodiment of theinvention.

FIG. 17 conceptually illustrates typical ray paths for a waveguidedisplay in accordance with an embodiment of the invention.

FIG. 18 conceptually illustrates a waveguide display in which thewaveguide supports a curved optical surface in accordance with anembodiment of the invention.

FIG. 19 conceptually illustrates a waveguide display in which thewaveguide supports upper and lower curved optical surfaces in accordancewith an embodiment of the invention.

FIG. 20 conceptually illustrates a waveguide display in which thewaveguide supports a curved optical surface and an input image isprovided using a pixel array predistorted to compensate for aberrationsintroduced by the curved optical surface in accordance with anembodiment of the invention.

FIG. 21 conceptually illustrates a waveguide display in which thewaveguide supports a curved optical surface and an input image isprovided using a pixel array supported by a curved substrate andpredistorted to compensate for aberrations introduced by the curvedoptical surface in accordance with an embodiment of the invention.

FIG. 22 is a flow chart conceptually illustrating a method forprojecting image light for view using a waveguide containingS-diffracting and P-diffracting gratings in accordance with anembodiment of the invention.

FIG. 23 is a flow chart conceptually illustrating a method forprojecting image light for view using a waveguide supporting an opticalprescription surface and containing S-diffracting and P-diffractinggratings in accordance with an embodiment of the invention.

FIG. 24A conceptually illustrates a portion of a pixel pattern havingrectangular elements of differing size and aspect ratio for use in anemissive display panel in accordance with an embodiment of theinvention.

FIG. 24B conceptually illustrates a portion of a pixel pattern havingPenrose tiles for use in an emissive display panel in accordance with anembodiment of the invention.

FIG. 24C conceptually illustrates a portion of a pixel pattern havinghexagonal elements for use in an emissive display panel in accordancewith an embodiment of the invention.

FIG. 24D conceptually illustrates a portion of a pixel pattern havingsquare elements for use in an emissive display panel in accordance withan embodiment of the invention.

FIG. 24E conceptually illustrates a portion of a pixel pattern havingdiamond-shaped elements for use in an emissive display panel inaccordance with an embodiment of the invention.

FIG. 24F conceptually illustrates a portion of a pixel pattern havingisosceles triangular elements for use in an emissive display panel inaccordance with an embodiment of the invention.

FIG. 24G conceptually illustrates a portion of a pixel pattern havinghexagonal elements with horizontally biased aspect ratios for use in anemissive display panel in accordance with an embodiment of theinvention.

FIG. 24H conceptually illustrates a portion of a pixel pattern havingrectangular elements with horizontally biased aspect ratios for use inan emissive display panel in accordance with an embodiment of theinvention.

FIG. 24I conceptually illustrates a portion of a pixel pattern havingdiamond shaped elements with horizontally biased aspect ratios for usein an emissive display panel in accordance with an embodiment of theinvention.

FIG. 24J conceptually illustrates a portion of a pixel pattern havingtriangles with horizontally biased aspect ratios for use in an emissivedisplay panel in accordance with an embodiment of the invention.

FIG. 25 conceptually illustrates a portion of a pixel pattern havingdiamond shaped elements in which different pixels can have differentemission characteristics in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

There is a growing interest in the use of various gratings on waveguidesin order to provide a variety of functions. These gratings include anglemultiplexed gratings, color multiplexed gratings, fold gratings, dualinteraction gratings, rolled K-vector gratings, crossed fold gratings,tessellated gratings, chirped gratings, gratings with spatially varyingrefractive index modulation, gratings having spatially varying gratingthickness, gratings having spatially varying average refractive index,gratings with spatially varying refractive index modulation tensors, andgratings having spatially varying average refractive index tensors. Inspecific examples, gratings for diffraction of various polarizations oflight (e.g. S-polarized light and P-polarized light) may be beneficial.It would be specifically advantageous to have a grating which diffractseither S-polarized light or P-polarized light. Specific applications forthis technology include waveguide-based displays such as augmentedreality displays and virtual reality displays. One example is inputgratings which may be used to input one or both of S-polarized light orP-polarized light into the waveguide. However, in many cases, it wouldbe advantageous to have a grating which diffracts either S-polarizedlight and P-polarized light. For example, waveguide displays usingunpolarized light sources such as OLED light sources produce bothS-polarized and P-polarized light and thus it would be advantageous tohave gratings which can diffract both S-polarized and P-polarized light.

One specific class of gratings includes surface relief gratings (SRGs)which may be used to diffract either P-polarized light or S-polarizedlight. Another class of gratings are surface relief Bragg gratings(SBGs) which are normally P-polarization selective, leading to a 50%efficiency loss with unpolarized light sources such as organic lightemitting diodes (OLEDs) and light emitting diodes (LEDs). Combining amixture of S-polarization diffracting and P-polarization diffractinggratings may provide a theoretical 2× improvement over waveguides usingP-diffracting gratings only. Thus, it would be advantageous to have ahigh efficiency S-polarization diffraction grating. In many embodiments,an S-polarization diffracting grating can be provided by a Bragg gratingformed in a holographic photopolymer. In some embodiments, anS-polarization diffracting grating can be provided by a Bragg gratingformed in a holographic polymer dispersed liquid crystal (HPDLC) withbirefringence altered using an alignment layer or other processes forrealigning the liquid crystal (LC) directors. In several embodiments, anS-polarization diffracting grating can be formed using liquid crystals,monomers, and other additives that naturally organize into S-diffractinggratings under phase separation. In some embodiments, these HPDLCgratings may form deep SRGs which have superior S-polarizationdiffraction efficiency.

One class of deep SRGs are polymer-air SRGs or evacuated Bragg gratings(EBGs) which may exhibit high S-diffraction efficiency (up to 99%) andlow P-diffraction efficiency and may be implemented as input gratingsfor waveguides. Such gratings can be formed by removing the liquidcrystal from SBGs formed from holographic phase separation of a liquidcrystal and monomer mixture. Deep SRGs formed by such a processtypically have a thickness in the range 1-3 micrometers with a Braggfringe spacing 0.35 to 0.80 micrometers. In some embodiments, the ratioof grating depth to Bragg fringe spacing may be 1:1 to 5:1. As canreadily be appreciated, such gratings can be formed with differentdimensions depending on the specific requirements of the givenapplication. Examples of how the thickness of SRGs may yield differentresultant diffraction efficiencies are described in connection withFIGS. 6-8.

In many embodiments, the condition for a deep SRGs is characterized by ahigh grating depth to fringe spacing ratio. In some embodiments, thecondition for the formation of a deep SRGs is that the grating depth isapproximately twice the grating period. Modelling such deep SRGs usingthe Kogelnik theory can give reasonably accurate estimates ofdiffraction efficiency, avoiding the need for more advanced modelling,which typically entails the numerical solution of Maxwell's equations.The grating depths that can be achieved using liquid crystal removalfrom HPDLC gratings greatly surpass those possible using conventionalnanoimprint lithographic methods, which cannot achieve the conditionsfor deep SRGs (typically providing only 250-300 nm depth for gratingperiods 350-460 nm). (Pekka Äyräs, Pasi Saarikko, Tapani Levola, “Exitpupil expander with a large field of view based on diffractive optics”,Journal of the SID 17/8, (2009), pp 659-664). It should be emphasizedhere that, although the S-polarization diffracting deep SRGs areemphasized within the present application, deep SRGs can, as will bediscussed below, provide a range of polarization responsecharacteristics depending on the thickness of the grating prescriptionand, in particular, the grating depth. As such, deep SRGs can beimplemented in a variety of different applications.

The literature supports equivalence of deep SRGs and Bragg gratings. Onereference (Kiyoshi Yokomori, “Dielectric surface-relief gratings withhigh diffraction efficiency” Applied Optics; Vol. 23; Issue 14; (1984);pp. 2303-2310), discloses the investigation of the diffractionproperties of dielectric surface-relief gratings by solving Maxwell'sequations numerically. The diffraction efficiency of a grating with agroove depth about twice as deep as the grating period was found to becomparable with the efficiency of a volume phase grating. The modellingby Yokomori predicted that dielectric surface-relief gratingsinterferometrically recorded in photoresist can possess a highdiffraction efficiency of up to 94% (throughput efficiency 85%). Theequivalence of deep SRGs and Bragg gratings is also discussed in anotherarticle by Golub (M. A. Golub, A. A. Friesem, L. Eisen “Bragg propertiesof efficient surface relief gratings in the resonance domain”, OpticsCommunications; 235; (2004); pp 261-267). A further article by Gerritsendiscusses the formation of Bragg-like SRGs in photoresist (Gerritsen HJ, Thornton D K, Bolton S R; “Application of Kogelnik's two-wave theoryto deep, slanted, highly efficient, relief transmission gratings”Applied Optics; Vol. 30; Issue 7; (1991); pp 807-814).

Many embodiments of this disclosure provide for methods of making SRGssuch as deep SRGs that can offer very significant advantages overnanoimprint lithographic process particle for slanted gratings. Bragggratings of any complexity can be made using interference or master andcontact copy replication. In some embodiments, after removing the LC,the SRGs can be back filled with a material with different properties tothe LC. This allows a Bragg grating with modulation properties that arenot limited by the grating chemistry needed for grating formation.

In some embodiments the back fill material may not be a LC material. Insome embodiments, the back fill material may have a higher index ofrefraction than air which may increase the angular bandwidth of awaveguide. In several embodiments, the deep SRGs can be partiallybackfilled with LC to provide a hybrid SRG/Bragg grating. Alternatively,in some embodiments, the refill step can be avoided by removing just aportion of the LC from the LC rich regions of the HPDLC to provide ahybrid SRG/Bragg grating. The refill approach has the advantage that adifferent LC can be used to form the hybrid grating. The materials canbe deposited using an inkjet deposition process.

In some embodiments, the methods described herein may be used to createphotonic crystals. Photonic crystals may be implemented to create a widevariety of diffracting structures including Bragg gratings. Bragggratings may be used as diffraction gratings to provide functionalityincluding but not limited to input gratings, output gratings, beamexpansion gratings, diffract more than one primary color. A photoniccrystal can be a three-dimensional lattice structure that can havediffractive capabilities not achievable with a basic Bragg grating.Photonic crystals can include many structures including all 2-D and 3-DBravais lattices. Recording of such structures may benefit from morethan two recording beams.

In some embodiments, waveguides incorporating photonic crystals can bearranged in stacks of waveguides, each having a grating prescription fordiffracting a unique spectral bandwidth. In many embodiments, a photoniccrystal formed by liquid crystal extraction provide a deep SRG. In manyembodiments, a deep SRG formed using a liquid crystal extraction processcan typically have a thickness in the range 1-3 micron with a Braggfringe spacing 0.35 micron to 0.80 micron. In many embodiments, thecondition for a deep SRG is characterized by a high grating depth tofringe spacing ratio. In some embodiments the condition for theformation of a deep SRG is that the grating depth can be approximatelytwice the grating period. It should be emphasized here that, althoughS-polarization diffracting deep SRGs are of main interest in the presentapplication, deep SRGs can, as will be discussed below, provide a rangeof polarization response characteristics depending on the thickness ofthe grating prescription and, in particular, the grating depth. DeepSRGs can also be used in conjunction with conventional Bragg gratings toenhance the color, uniformity and other properties of waveguidedisplays.

Deep SRGs have been fabricated in glassy monomeric azobenzene materialsusing laser holographic exposure (O. Sakhno, L. M. Goldenberg, M.Wegener, J. Stumpe, “Deep surface relief grating inazobenzene-containing materials using a low intensity 532 nm laser”,Optical Materials: X, 1, (2019), 100006, pp 3-7. The Sakhno referencealso discloses how SRGs can be recorded in a holographic photopolymerusing two linearly orthogonally polarized laser beams.

The disclosure provides a method for making a surface relief gratingthat can offer very significant advantages over nanoimprint lithographicprocess particularly for slanted gratings. Bragg gratings of anycomplexity can be made using interference or master and contact copyreplication. In some embodiments after removing the LC the SRG can beback filled with a material with different properties to the LC. Thisallows a Bragg grating with modulation properties that are not limitedby the grating chemistry needed for grating formation. In someembodiments the SRGs can be partially backfilled with LC to provide ahybrid SRG/Bragg grating. Alternatively, in some embodiments, the refillstep can be avoided by removing just a portion of the LC from the LCrich regions of the HPDLC to provide a hybrid SRG/Bragg grating. Therefill approach has the advantage that a different LC can be used toform the hybrid grating. The materials can be deposited using an inkjetprocess as disclosed in earlier filings by the inventors. In someembodiments, the refill material may have a higher index of refractionthan air which may increase diffraction efficiency of the grating.

While this disclosure has been made in the context of fabricating deepSRGs, it is appreciated that many other grating structures may beproduced using the techniques described herein. For example, any type ofSRG including SRGs in which the grating depth is smaller than thegrating frequency (e.g. Raman-Nath gratings) may be fabricated as well.

FIGS. 1A-1D illustrate a processing apparatus that can be used in amethod for fabricating deep SRGs or EBGs in accordance with anembodiment. FIG. 1A conceptually illustrates an apparatus 190A that canbe used in a step of a method for fabricating a surface relief gratingin which a mixture 191 of monomer and liquid crystal deposited on atransparent substrate 192 is exposed to holographic exposure beams193,194, in accordance with an embodiment of the invention. In someexamples, the mixture may also include at least one of a photoinitiator,a coinitiator, a multifunctional thiol, adhesion promoter, surfactant,and/or additional additives. In some embodiments, the monomer may beisocyanate-acrylate based or thiolene based. In some embodiments, theliquid crystal may be a full liquid crystal mixture or a liquid crystalsingle only including a portion of a full liquid crystal mixture.Various examples of liquid crystal singles include one or both ofcyanobiphenyls or pentylcyanobiphenyls. In some embodiments, liquidcrystal may be replaced with another substance that phase separates withthe monomer during exposure to create polymer rich regions and substancerich regions. Advantageously, the substance and liquid crystal singlesmay be a cost-effective substitute to full liquid crystal mixtures whichare removed at a later step as described below.

FIG. 1B conceptually illustrates an apparatus 190B that can be used in astep of a method for fabricating a surface relief grating from an HPDLCBragg grating 195 formed on a transparent substrate using theholographic exposure beams, in accordance with an embodiment of theinvention. The holographic exposure beams may transform the monomer intoa polymer in some areas. The holographic exposure beams may includeintersecting recording beams and include alternating bright and darkillumination regions. A polymerization-driven diffusion process maycause the diffusion of monomers and LC in opposite directions, with themonomers undergoing gelation to form polymer-rich regions (in the brightregions) and the liquid crystal becoming trapped in a polymer matrix toform liquid crystal rich regions (in the dark regions).

FIG. 1C conceptually illustrates an apparatus 190C that can be used in astep of a method for fabricating a deep polymer surface relief grating196 or EBG in which liquid crystal is removed from an HPDLC Bragggrating of FIG. 1B to form a polymer surface relief grating inaccordance with an embodiment of the invention. Advantageously, apolymer surface relief grating 196 may include a large depth with acomparatively small grating period in order to form a deep SRG. Theliquid crystal may be removed by washing with a solvent such asisopropyl alcohol (IPA). The solvent should be strong enough to washaway the liquid crystal but weak enough to maintain the polymer. In someembodiments, the solvent may be chilled below room temperature beforewashing the grating. FIG. 1D conceptually illustrates an apparatus 190Dthat can be used in a step of a method for fabricating a polymer surfacerelief grating in which the polymer surface relief grating is coveredwith a protective layer 197 in accordance with an embodiment of theinvention.

FIG. 2 conceptually illustrates a method for forming deep SRGs from aHPDLC Bragg grating formed on a transparent substrate in accordance withan embodiment. As shown, the method 200 of forming deep SRGs or EBGs isprovided. Referring to the flow diagram, the method 200 includesproviding (201) a mixture of at least one monomer and at least oneliquid crystal. The at least one monomer may include anisocyanate-acrylate monomer or thiolene. In some embodiments, the atleast one liquid crystal may be a full liquid crystal mixture or may bea liquid crystal single which may include only a portion of the liquidcrystal mixture such as a single component of the liquid crystalmixture. In some embodiments, the at least one liquid crystal may besubstituted for a solution which may phase separate with the monomerduring exposure. The criteria for such a solution may include ability tophase separate with the monomer during exposure, ease of removal aftercuring and during washing, and ease of handing. Example substitutesolutions include solvents, non-reactive monomers, inorganics, andnanoparticles.

Providing the mixture of the monomer and the liquid crystal may alsoinclude mixing one or more of the following with the at least onemonomer and the liquid crystal: initiators such as photoinitiators orcoinitiators, multifunctional thiol, dye, adhesion promoters,surfactants, and/or additional additives such as other cross linkingagents. This mixture may be allowed to rest in order to allow thecoinitiator to catalyze a reaction between the monomer and the thiol.The rest period may occur in a dark space or a space with red light(e.g. infrared light) at a cold temperature (e.g. 20° C.) for a periodof approximately 8 hours. After resting, additional monomers may bemixed into the monomer. This mixture may be then strained or filteredthrough a filter with a small pore size (e.g. 0.45 μm pore size). Afterstraining, this mixture may be stored at room temperature in a darkspace or a space with red light before coating.

Next, a transparent substrate can be provided (202). In certainembodiments, the transparent substrate may be a glass substrate or aplastic substrate. A layer of the mixture can be deposited or coated(203) onto a surface of the substrate. In some embodiments, the mixtureis sandwiched between the transparent substrate and another substrateusing glass spacers to maintain internal dimensions. A non-stick coatingmay be applied to the other substrate before the mixture is sandwiched.The non-stick coating may include a fluoropolymer such as OPTOOL UD509(produced by Daikin Chemicals), Dow Corning 2634, Fluoropel (produced byCytonix), and EC200 (produced by PPG Industries, Inc). Holographicrecording beams can be applied (204) to the mixture layer. Theholographic recording beams may be a two-beam interference pattern whichmay cause phase separation of the LC and the polymer. In response to theholographic recording beam, the liquid monomer changes to a solidpolymer whereas the neutral, non-reactive substance (e.g. LC) diffusesduring holographic exposure in response to a change in chemicalpotential driven by polymerization. While LC may be one implementationof the neutral, non-reactive substance, other substances may also beused. The substance and the monomer may form a miscible mixture prior tothe holographic exposure and become immiscible upon holographicexposure.

After applying the holographic recording beams, the mixture may becured. The curing process may include leaving the mixture underlow-intensity white light for a period of time until the mixture fullycures. The low intensity white light may also cause a photo-bleach dyeprocess to occur. Thus, a HPDLC grating having alternating polymer richand liquid crystal rich regions can be formed (205). In someembodiments, the curing process may occur in two hours or less. Aftercuring, one of the substrates may be removed exposing the HPDLC grating.Advantageously, the non-stick coating may allow the other substrate tobe removed with the HPDLC grating remaining.

HPDLC grating may include alternating sections of liquid crystal richregions and polymer regions. The liquid crystal in the liquid crystalrich regions can be removed (206) to form polymer surface reliefgratings or EBGs which may be used as deep SRGs. The liquid crystal maybe removed by gently immersing the grating into a solvent such as IPA.The IPA may be chilled and may be kept at a temperature lower than roomtemperature while the grating is immersed in the IPA. The grating isthen removed from the solvent and dried. In some embodiments, thegrating is dried using a high flow air source such as compressed air.After the LC is removed from the grating, a polymer-air surface reliefBragg grating is formed.

As shown in FIGS. 1A-1D, the formed surface relief grating can furtherbe covered with a protective layer. In some instances, the protectivelayer may be a moisture and oxygen barrier with scratch resistancecapabilities. In some instances, the protective layer may be a coatingthat does not fill in air gap regions where LC that was removed onceexisted. The coating may be deposited using a low temperature process.In some implementations, the protective layer may have anti-reflective(AR) properties. The coating may be a silicate or silicon nitride. Thecoating process may be preformed by a plasma assisted chemical vapordeposition (CVD) process such as a nanocoating process. The coating maybe a parylene coating. The protective layer may be a glass layer. Avacuum or inert gas may fill the gaps where LC that was removed onceexisted before the protective layer is implemented. In some embodiments,the coating process may be integrated with the LC removal process (206).For example, a coating material may be mixed with the solvent which isused to wash the LC from the grating.

FIG. 3 illustrates a cross sectional schematic view of an exemplaryembodiment of a polymer-air surface relief Bragg grating 3000implemented on a waveguide 3002. The polymer-air surface relief Bragggrating 3000 includes periodic polymer sections 3004 a. Adjacent polymersections sandwich air sections 3004 b. The air sections 3004 b aresandwiched by polymer sections 3004 a. The air sections 3004 b andpolymer sections 3004 a have different indexes of refraction.Advantageously, the polymer-air surface relief Bragg grating 3000 may beformed with a high grating depth 3006 a to Bragg fringe spacing 3006 bratio which may create a deep SRG. As discussed previously, deep SRGsmay exhibit many beneficial qualities such as high S-diffractionefficiency which may not be present within the typical SRGs.

In one example, a polymer-air surface relief Bragg grating 3000 may havea Bragg fringe spacing 3006 b of 0.35 μm to 0.8 μm and a grating depthof 1 μm to 3 μm. In some embodiments, the polymer sections 3004 a mayinclude at least some residual liquid crystal when the liquid crystal isnot completely removed during step 206 described in connection with FIG.2. In some embodiments, the presence of residual LC within the polymerrich regions may increase refractive index modulation of the finalpolymer SRG. In some embodiments, the air sections 3004 b may includesome residual liquid crystal if the liquid crystal is not completelyremoved during step 206 from these air sections 3004 b. In someembodiments, by leaving some residual liquid crystal within the airsections 3004 b, a hybrid grating as described in connection with FIGS.4-5 may be created.

As discussed above, in many the embodiments, the invention also providesa method for fabricating a hybrid surface relief/Bragg grating. FIG. 4Aconceptually illustrates an apparatus 210A that can be used in a step ofa method for fabricating hybrid surface relief gratings (hybrid SRGs) inwhich a mixture 211 of monomer and liquid crystal deposited on atransparent substrate 212 is exposed to holographic exposure beams213,214, in accordance with an embodiment of the invention. FIG. 4Bconceptually illustrates an apparatus 210B that can be used in a step ofa method for fabricating hybrid SRGs from an HPDLC Bragg grating 215formed on the transparent substrate using the holographic exposure beamsin accordance with an embodiment of the invention. FIG. 4C conceptuallyillustrates an apparatus 210C that can be used in a step of a method forfabricating a surface relief grating in which liquid crystal is removedfrom an HPDLC Bragg grating to form polymer-air SRGs 216 in accordancewith an embodiment of the invention. These polymer-air SRGs 216 or EBGsmay be deep SRGs. It is appreciated that the steps illustrated in anddescribed in connection with FIGS. 4A-4C roughly correspond to the stepsillustrated in and described in connection with FIGS. 2A-2C in theprocess to create a polymer-air SRG and thus the previous descriptionwill be applicable to FIGS. 4A-4C.

In addition, FIG. 4D conceptually illustrates an additional step whichmay be performed to create a hybrid grating. The apparatus 210D can beused in a step of a method for fabricating a surface relief grating inwhich a surface relief grating is at least partially refilled withliquid crystal to form a hybrid SRGs 217, in accordance with anembodiment of the invention. The refilled liquid crystal may be ofdifferent consistency to the previously removed liquid crystal that waspreviously removed in FIG. 4C. Further, it is appreciated that theliquid crystal removed in FIG. 3C may only be partially removed in analternative method to forming hybrid SRGs 217. In addition, FIG. 4Econceptually illustrates an apparatus 210E can be used in a step of amethod for fabricating a surface relief grating in which hybrid SRGs 217formed in the step illustrated in FIG. 4D is covered with a protectivelayer 218, in accordance with an embodiment of the invention.

FIG. 5 is a flowchart showing an exemplary method for forming a hybridsurface relief-Bragg grating from a HPDLC Bragg grating formed on atransparent substrate in accordance with an embodiment of the invention.As shown, the method 220 of forming hybrid surface relief-Bragg gratingis provided. Referring to the flow diagram, method 220 includesproviding (221) a mixture of at least one monomer and at least oneliquid crystal. The at least one monomer may include anisocyanate-acrylate monomer. Providing the mixture of the monomer andthe liquid crystal may also include mixing one or more of the followingwith the at least one monomer and the liquid crystal: photoinitiator,coinitiator, multifunctional thiol, and/or additional additives. Thismixture may be allowed to rest in order to allow the coinitiator tocatalyze a reaction between the monomer and the thiol. The rest periodmay occur in a dark space or a space with red light (e.g. infraredlight) at a cold temperature (e.g. 20° C.) for a period of approximately8 hours. After resting, additional monomers may be mixed into themonomer. This mixture may be then strained or filtered through a filterwith a small pore size (e.g. 0.45 μm pore size). After straining thismixture may be stored at room temperature in a dark space or a spacewith red light before coating.

Next, a transparent substrate can be provided (222). In certainembodiments, the transparent substrate may be a glass substrate or aplastic substrate. A non-stick coating may be applied to the transparentsubstrate before the mixture is coated on the substrate. A layer of themixture can be deposited (223) onto a surface of the substrate. In someembodiments, the mixture is sandwiched between the transparent substrateand another substrate using glass spacers to maintain internaldimensions. Holographic recording beams can be applied (224) to themixture layer. The holographic recording beams may be a two-beaminterference pattern which may cause phase separation of the LC and thepolymer. After applying the holographic recording beams, the mixture maybe cured. The curing process may include leaving the mixture underlow-intensity white light for a period of time under the mixture fullycures. The low intensity white light may also cause a photo-bleach dyeprocess to occur. Thus, an HPDLC grating having alternating polymer richand liquid crystal rich regions can be formed (225). In someembodiments, the curing process may occur in 2 hours or less. Aftercuring, one of the substrates may be removed exposing the HPDLC grating.

HPDLC grating may include alternating sections of liquid crystal richregions and polymer regions. The liquid crystal in the liquid crystalrich regions can be removed (226) to form polymer surface reliefgratings or EBGs which is a form of deep SRGs. The liquid crystal may beremoved by gently immersing the grating into a solvent such as isopropylalcohol (IPA). The IPA may be kept at a lower temperature while thegrating is immersed in the IPA. The grating is them removed from thesolvent and dried. In some embodiments, the grating is dried using ahigh flow air source such as compressed air. After the LC is removedfrom the grating, a polymer-air surface relief Bragg grating is formed.The steps 221-226 of FIG. 5 roughly correspond to the steps described inconnection with FIG. 2 in creating a polymer-air SRG and thus thesedescriptions are applicable to FIG. 5.

Further, method 220 includes at least partially refilling (227) clearedliquid crystal rich regions with liquid crystal to form hybrid SRGs. Therefilled liquid crystal may be of different consistency to thepreviously removed liquid crystal that was previously removed in step226. Further, it is appreciated that the liquid crystal removed in step226 may only be partially removed in an alternative method to forminghybrid SRGs. Advantageously, hybrid SRGs may provide the ability totailor specific beneficial characteristics of the SRGs. One particularcharacteristic that may be improved by the inclusion of at least someliquid crystal within the SRGs is a decrease in haze properties.

As shown in FIG. 4E, the formed surface relief grating can further becovered with a protective layer. In some instances, the protective layermay be a moisture and oxygen barrier with scratch resistancecapabilities. In some instances, the protective layer may be a coatingthat does not fill in air gap regions where LC that was removed onceexisted. The coating may be deposited using a low temperature process.In some implementations, the protective layer may have anti-reflective(AR) properties. The coating may be a silicate or silicon nitride. Thecoating process may be preformed by a plasma assisted chemical vapordeposition (CVD) process such as a plasmatreat nanocoating process. Thecoating may be a parylene coating. The protective layer may be a glasslayer. A vacuum or inert gas may fill the gaps where LC that was removedonce existed before the protective layer is implemented. In someembodiments, the coating process may be integrated with the LC removalprocess (226). For example, a coating material may be mixed with thesolvent which is used to wash the LC from the grating. In someimplementations, the coating material may be a material with a lower orhigher refractive index than the polymer and fill the spaces betweenadjacent polymer portions. The refractive index difference between thepolymer and the coating material may allow the polymer SRGs to continueto diffract.

Although FIGS. 1-5 illustrate specific methods and apparatus for formingdeep SRGs and hybrid surface relief/Bragg gratings, variousmanufacturing methods implementing different steps or modifications ofsuch steps can be utilized. As can readily be appreciated, the specificprocess utilized can depend on the specific requirements of the givenapplication. For example, many embodiments utilize another grating as aprotective layer.

Hybrid SRG/Bragg gratings with shallow SRG structures may lead to lowSRG diffraction efficiencies. The methods disclosed in the presentdisclosure allows for more effective SRG structures to be formed byoptimizing the depth of the liquid crystal in the liquid crystal richregions such that the SRGs has a high depth to grating pitch ratio whileallowing the Bragg grating to be sufficiently thick for efficientdiffraction. In many embodiments, the Bragg grating component of thehybrid grating can have a thickness in the range 1-3 micrometer. In someembodiments, the SRG component of the hybrid grating can have athickness in the range 0.25-3 micrometer. The initial HPDLC gratingwould have a thickness equal to the sum of the final SRG and Bragggrating components. As can readily be appreciated, the thickness ratioof the two grating components can depend on the waveguide application.In some embodiments, the combination of an SRG with a Bragg grating maybe used to fine-tune angular bandwidth of the grating structure. In somecases, the SRG can increase the angular bandwidth of the gratingstructure.

In many embodiments, in the hybrid SRGs illustrated in FIGS. 4A-4E, therefill depth of the liquid crystal regions of the grating can be variedacross the grating to provide spatially varying relative SRG/Bragggrating strengths. In some embodiments, during the liquid crystalremoval and refill as defined in steps 206, 226, and 227, the liquidcrystal in the liquid crystal rich grating regions can be totally orpartially removed. In several embodiments, the liquid crystal used torefill or partially refill the liquid crystal-cleared regions can have adifferent chemical composition to the liquid crystal used to form theinitial HPDLC grating. In various embodiments, a first liquid crystalwith phase separation properties compatible with the monomer can bespecified to provide a HPDLC grating with optimal modulation and gratingdefinitions while a second refill liquid crystal can be specified toprovide desired index modulation properties in the final hybrid grating.In a number of embodiments, the Bragg portion of the hybrid grating canbe switchable with electrodes applied to surfaces of the substrate andthe cover layer. In many embodiments, the refill liquid crystals cancontain additives which may include but are not limited to the featuresof improving switching voltage, switching time, polarization,transparency, and other parameters. A hybrid grating formed using arefill process would have the further advantages that the LC would forma continuum (rather than an assembly of LC droplets), thereby reducinghaze.

While deep SRGs, EBGs, and/or hybrid SRGs may be described in thecontext of S-diffracting gratings and P-diffracting gratings, thesegratings have applicability in many other grating types. These includebut are not limited to angle multiplexed gratings, color multiplexedgratings, fold gratings, dual interaction gratings, rolled K-vectorgratings, crossed fold gratings, tessellated gratings, chirped gratings,gratings with spatially varying refractive index modulation, gratingshaving spatially varying grating thickness, gratings having spatiallyvarying average refractive index, gratings with spatially varyingrefractive index modulation tensors, and gratings having spatiallyvarying average refractive index tensors. Further, deep SRGs, EBGs,and/or hybrid SRGs may be switchable or non-switchable gratingsdepending on their specific implementation. Deep SRGs, EBGs, and/orhybrid SRGs may be fabricated on a plastic substrate or a glasssubstrate. These gratings may also be fabricated on one substrate andtransferred to another substrate.

Discussion of Various Implementations of Deep SRGs or EBGs

In many embodiments, deep SRGs can provide a means for controllingpolarization in a waveguide. SBGs are normally P-polarization selective,leading to a 50% efficiency loss with unpolarized light sources such asOLEDs and LEDs. Hence, combining S-polarization diffracting andP-polarization diffracting gratings can provide a theoretical 2×improvement over waveguides using P-diffracting gratings only. In someembodiments, an S-polarization diffracting grating can be provided by aBragg grating formed in a conventional holographic photopolymer. In someembodiments an S-polarization diffracting grating can be provided by aBragg grating formed in a HPDLC with birefringence altered using analignment layer or other process for realigning the liquid crystaldirectors. In some embodiments, an S-polarization diffracting gratingcan be formed using liquid crystals, monomers and other additives thatnaturally organize into S-diffracting gratings under phase separation.In many embodiments, an S-polarization diffracting grating can beprovided by SRGs. Using the processes described above, a deep SRGexhibiting high S-diffraction efficiency (up to 99%) and lowP-diffraction efficiency can be formed by removing the liquid crystalfrom SBGs formed from holographic phase separation of a liquid crystaland monomer mixture.

Deep SRGs can also provide other polarization response characteristics.Several prior art theoretical studies such as an article by Moharam(Moharam M. G. et al. “Diffraction characteristics of photoresistsurface-relief gratings”, Applied Optics, Vol. 23, page 3214, Sep. 15,1984) point to deep surface relief gratings having both S and Psensitivity with S being dominant. In some embodiments, deep SRGsdemonstrate the capability of providing an S-polarization response.However, deep SRGs may also provide other polarization responsecharacteristics. In many embodiments, deep surface relief gratingshaving both S and P sensitivity with S being dominant are implemented.In some embodiments, the thickness of the SRG can be adjusted to providea variety of S and P diffraction characteristics. In severalembodiments, diffraction efficiency can be high for P across a spectralbandwidth and angular bandwidth and low for S across the same spectralbandwidth and angular bandwidth. In number of embodiments, diffractionefficiency can be high for S across the spectral bandwidth and angularbandwidth and low for P across the same spectral bandwidth and angularbandwidth. In some embodiments, high efficiency for both S and Ppolarized light can be provided. A theoretical analysis of an SRG ofrefractive index 1.6 immersed in air (hence providing an average gratingindex of 1.3) of period 0.48 micron, with a 0 degrees incidence angleand 45 degree diffracted angle for a wavelength of 0.532 micron is shownin FIGS. 5-7. FIG. 5 is a graph showing calculated P-polarized andS-polarized diffraction efficiency versus incidence angle for a1-micrometer thickness deep surface relief grating, demonstrating thatin this case high S and P response can be achieved. FIG. 6 is a graphshowing calculated P-polarized and S-polarized diffraction efficiencyversus incidence angle for a 2-micrometer thickness deep surface reliefgrating, demonstrating that in this case the S-polarization response isdominant over most of the angular range of the grating. FIG. 7 is agraph showing calculated P-polarized and S-polarized diffractionefficiency versus incidence angle for a 3-micrometer thickness,demonstrating that in this case the P-polarization response is dominantover a substantial portion of the angular range of the grating.

In many embodiments, a photonic crystal can be a reflection Bragggrating or deep SRG formed by a LC extraction process. A reflection deepSRG made using phase separation followed by LC subtraction can enablewide angular and spectral bandwidth. In many embodiments replacing thecurrent input SBG with a reflection photonic crystal can be used toreduce the optical path from a picture generation unit (PGU) to awaveguide. In some embodiments, a PGU pupil and the waveguide can be incontact. In many embodiments, the reflection deep SRG can beapproximately 3 microns in thickness. The diffracting properties of anLC extracted Bragg grating mainly result from the index gap between thepolymer and air (not from the depth of the grating as in the case of atypical SRG).

Discussion of Thiol Additives within Initial Mixture

FIGS. 9A and 9B illustrate comparative scattering electron microscopy(SEM) images of example mixtures used to fabricate polymer-air SRGs. Asdiscussed previously, the monomer within the initial mixture may beacrylate or thiolene based. It has been discovered that with somemonomers such as acrylate based monomers, after holographic exposure,during washing, the solvent not only removes liquid crystal material butalso polymer which is unideal. It has been discovered that amultifunctional thiol additive may solve this issue by strengthening thepolymer and thus allowing it to be strong enough to withstand thesolvent wash. Without limiting to any particular theory, thiol additivemay improve the mechanical strength of formulations consisting of lowfunctionality acrylate monomers which tend to form mechanically weakpolymers due to reduced cross-linking. Acrylate monomer formulations maybe advantageous because they may exhibit high diffraction efficiencywith lower haze. Thus, adding thiol could allow Acrylate monomerformations to be a viable option in fabrication of polymer SRGs.

There may be a trade-off between phase separation, grating formation,and mechanical strength between different formulations. Gratingformation may benefit from mixtures that contain low functionalitymonomers that react slower, form fewer cross-linkages, and allow greaterdiffusion of non-reactive components (e.g. LC) during holographicexposure. Conversely, mixtures consisting of high functionality monomersmay exhibit better phase separation and polymer mechanical strength dueto greater cross-linking, but may react so rapidly that the non-reactivecomponents do not have sufficient time to diffuse and thus may exhibitlower diffraction efficiency as a result.

Without limitation to any particular theory, the thiol additives may getaround these limitations by reacting with acrylates orisocyanate-acrylates to form a loose scaffolding prior to holographicexposure. This scaffolding may improve the mechanical strength anduniformity of the cured polymer. Thus, the mechanical strength may betuned through slight adjustments of the thiol functionality andconcentration without significantly raising the average functionality ofthe monomer mixture and disrupting grating formation.

FIG. 9A illustrates an initial mixture whereas FIG. 9B illustrate acomparative mixture which includes 1.5 wt % thiol. However, other weightpercentages of thiol additive have been contemplated. For example, aweight percentage of thiol additive may be 1 to 4% or 1.5% to 3%. Insome embodiments, the multifunctional thiol may be trimethylolpropanetris(3-mercaptopropionate). Both FIGS. 9A and 9B include polymer denseregions 902 a/902 b and air regions 904 a/904 b. As illustrated, theadded thiol may produce a denser polymer structure within the polymerdense regions 902 a of FIG. 9B than the polymer dense regions 902 b ofFIG. 9A which may increase grating performance. It has been discoveredthat the weight percentage of thiol additive should be balanced in orderto provide stability within the polymer structure to withstand thesolvent wash however not to be rigid as to not allow the liquid crystalto be released during the solvent wash.

Comparison between HPDLC Grating Performance with Polymer-Air SRGPerformance

FIGS. 10A and 10B illustrates images of comparative examples of an HPDLCgrating and a polymer SRG or EBG. FIG. 10A illustrates performance foran example HPDLC grating where liquid crystal has not been removed. Thegrating of FIG. 10A includes a 20-30% P-diffraction efficiency whileexhibiting a nominal or almost 0% 5-diffraction efficiency. FIG. 10Billustrates performance of an example polymer-air SRG where the LC hasbeen removed. The grating of FIG. 10B includes a 18-28% P-diffractionefficiency while exhibiting a S-diffraction efficiency of 51-77%. Thus,polymer-air SRGs where LC has been removed demonstrate a comparativelyhigh S-diffraction efficiency while maintaining a comparableP-diffraction efficiency. Further, the grating of FIG. 10B includes aP-diffraction haze of 0.11-0.15% and a S-diffraction haze of 0.12-0.16%.

FIGS. 11A and 11B illustrates plots of comparative examples of an HPDLCgrating where liquid crystal has not been removed and a polymer SRG orEBG where liquid crystal has been removed. FIG. 11A illustrates theP-diffraction efficiency and 5-diffraction efficiency for an HPDLCgrating where liquid crystal remains. A first line 1102 a corresponds toP-diffraction efficiency and a second line 1104 a corresponds toS-diffraction efficiency. FIG. 11B illustrates the P-diffractionefficiency and S-diffraction efficiency for a polymer SRG or EBG whereliquid crystal has been removed. A first line 1102 b corresponds toP-diffraction efficiency and a second line 1104 b corresponds to Sdiffraction efficiency. As illustrated, S-diffraction efficiencydramatically increases after liquid crystal has been removed whileP-diffraction efficiency remains relatively similar.

In some embodiments, the ratio of S-diffraction efficiency toP-diffraction efficiency may be adjusted by using different gratingperiods, grating slant angles, and grating thicknesses.

Various Example Deep SRG Depths

FIGS. 12A and 12B illustrate various comparative examples ofP-diffraction and S-diffraction efficiencies with deep SRGs of variousdepths. Each of these plots show diffraction efficiency vs. angle. InFIG. 12A, the deep SRG has a depth of approximately 1.1 μm. The firstline 1102 a represents S-diffraction efficiency and the second line 1104a represents P-diffraction efficiency. As illustrated the peakS-diffraction efficiency is approximately 58% and the peak P-diffractionefficiency is 23%. It is noted that the haze for S-diffraction is 0.12%and haze for P-diffraction is 0.11% for this example. Such highdiffraction efficiency with low haze may make deep SRGs with a depth ofapproximately 1.1 μm particularly suitable for multiplexed gratings.

In FIG. 12B, the deep SRG has a depth of approximately 1.8 μm. The firstline 1102 b represents S-diffraction efficiency and the second line 1104b represents P-diffraction efficiency. As illustrated the peakS-diffraction efficiency is approximately 92% and the peak P-diffractionefficiency is 63%. It is noted that the haze for S-diffraction is 0.34%and haze for P-diffraction is 0.40% for this example. Thus, bothS-diffraction and P-diffraction efficiency increase dramatically with anincreased grating depth. It is noted that haze appears to increase withthe increased grating depth.

Various Example Initial LC Concentrations in Mixture

FIGS. 13A and 13B illustrate the results of a comparative study ofvarious EBGs with various initial LC concentrations in the initialmixture. FIG. 13A illustrates S-diffraction efficiency vs. angle. FIG.13B illustrates P-diffraction efficiency vs. angle. In FIG. 13A, a firstline 1202 a corresponds to 20% initial LC content, a second line 1204 acorresponds to 30% initial LC content, and a third line 1206 acorresponds to 40% initial LC content. In FIG. 13B, a first line 1202 bcorresponds to 20% initial LC content, a second line 1204 b correspondsto 30% initial LC content, and a third line 1206 b corresponds to 40%initial LC content. Table 1 illustrates a summary of various results ofthe comparative study.

TABLE 1 Initial LC Maximum Maximum Content in S-DiffractionP-Diffraction S-Diffraction P-Diffraction Mixture Efficiency EfficiencyHaze Haze 20%  10%  5% 0.10% 0.12% 30% ≥40% 18% 0.14% 0.13% 40% ≥55% 23%0.12% 0.11%

As is illustrated in FIGS. 13A and 13B and noted in Table 1, the maximumS-diffraction and maximum P-diffraction appear to both increase withhigher initial LC content while the S-diffraction haze and P-diffractionhaze stay approximately constant.

FIGS. 14A and 14B illustrate additional example S-diffraction andP-diffraction efficiencies for various initial LC concentrations. FIG.14A illustrates S-diffraction efficiency for various example EBGsincluding various initial LC contents. FIG. 14B illustrate P-diffractionefficiency for various example EBGs including various LC contents. Forboth FIGS. 14A and 14B, sequentially from top to bottom the linesrepresent: 32% LC content, 30% LC content, 28% LC content, 26% LCcontent, 24% LC content, 22% LC content, and 20% LC content. Asillustrated, the S-diffraction and P-diffraction efficiencies aredirectly related to the amount of LC content (e.g. higher LC contentyields higher S-diffraction and P-diffraction efficiencies).

Without being limited to any particular theory, the initial LC contentrelates to amount of phase separation between the LC and the monomerthat occurs during the holographic exposure process and polymerizationprocess. Thus, a higher LC content will increase the amount of LC richregions which are removed to make more air regions after washing. Theincreased air regions make greater refractive index differences (Δn)between the air regions (formerly liquid crystal rich regions) and thepolymer rich regions which increases both S-diffraction andP-diffraction efficiencies. In some embodiments, the average refractiveindex of the polymer SRGs may be adjusted by adjusting the initialneutral substance (e.g. LC) content, thereby either increasing ordecreasing the volume of polymer after removal of the neutral substance.Further, increasing the initial neutral substance content may impact themechanical strength. Thus, an increase or decrease in mechanicalstrengthener such as thiol additive may be used to balance out theincrease or decrease in mechanical strength.

Embodiments Including OLED Arrays as Image Generators

There is growing interest in the use of Organic Light Emitting Diode(OLED) arrays as image generators in waveguide displays. OLEDs have manyadvantages in waveguide display applications. As an emissive technology,OLEDs require no light source. OLEDs can be printed cost—effectivelyover large areas. Non-rectangular pixel array patterns can be printedonto curved or flexible substrates. As will be discussed below, theability to pre-distort a pixel array and create a curved focal planeadds a new design dimension that can enable compensation for guided beamwavefront distortions caused by curved waveguides and prescriptionlenses supported by a waveguide. OLEDs with resolutions of 4K×4K pixelsare currently available with good prospects of higher resolution in thenear term, offering a faster route to high resolution, wide FOV ARdisplays than can be provided by technologies such as Liquid Crystal onSilicon (LCoS) and Micro Electro Mechanical Systems (MEMS) devices suchas digital light processing (DLP) devices. Another significant advantageover LCoS is that OLEDs can switch in microseconds (compared withmilliseconds for LC devices).

OLEDs have certain disadvantages. In their basic form, OLEDs areLambertian emitters, which makes efficient light collection much morechallenging than with LCoS and DLP micro displays. The red, green, andblue spectral bandwidths of OLEDs are broader than those of LightEmitting Diodes (LEDs), presenting further light management problems inholographic waveguides. The most significant disadvantage of OLEDs isthat in waveguides using HPDLC gratings such as Switchable BraggGratings (SBGs), which tend to be P-polarization selective, half of theavailable light from the OLED is wasted. As such, many embodiments ofthe invention are directed towards waveguide displays for use withemissive unpolarized image sources that can provide high lightefficiency for unpolarized light and towards related methods ofmanufacturing such waveguide displays.

For the purposes of describing embodiments, some well-known features ofoptical technology known to those skilled in the art of optical designand visual displays have been omitted or simplified in order to notobscure the basic principles of the invention. Unless otherwise stated,the term “on-axis” in relation to a ray or a beam direction refers topropagation parallel to an axis normal to the surfaces of the opticalcomponents described in relation to the invention. In the followingdescription the terms light, ray, beam, and direction may be usedinterchangeably and in association with each other to indicate thedirection of propagation of electromagnetic radiation along rectilineartrajectories. The term light and illumination may be used in relation tothe visible and infrared bands of the electromagnetic spectrum. Parts ofthe following description will be presented using terminology commonlyemployed by those skilled in the art of optical design. As used herein,the term grating may encompass a grating comprised of a set of gratingsin some embodiments. For illustrative purposes, it is to be understoodthat the drawings are not drawn to scale unless stated otherwise.

Turning now to the drawings, methods and apparatus for providingwaveguide displays using emissive input image panels in accordance withvarious embodiments of the invention are illustrated. FIG. 15conceptually illustrates a waveguide display in accordance with anembodiment of the invention. As shown, the apparatus 100 includes awaveguide 101 supporting input 102 and output 103 gratings with highdiffraction efficiency for P-polarized light in a first wavelength bandand input 104 and output 105 gratings with high diffraction efficiencyfor S-polarized light in the first wavelength band.

The apparatus 100 further includes an OLED microdisplay 106 emittingunpolarized light with an emission spectral bandwidth that includes thefirst wavelength band and a collimation lens 107 for projecting lightfrom the OLED microdisplay 106 into a field of view. In the illustrativeembodiment, the S and P diffracting gratings 102-105 can be layered withno air gap required. In other embodiments, the grating layers can beseparated by an air gap or a transparent layer. The S and P diffractinggratings 102-105 may be the deep SRGs or EBGs described above.

FIG. 16 conceptually illustrates a waveguide display in accordance withan embodiment of the invention in which the P-diffracting andS-diffracting gratings are disposed in separate air-spaced waveguidelayers. As shown, the apparatus 110 comprises upper 111 and lower 112waveguide layers (supporting the gratings 102,103 and 104,105,respectively) separated by an air gap 113. The gratings 102,103 and104,105 may be the deep SRGs and EBGs described above.

FIG. 17 conceptually illustrates typical ray paths in a waveguidedisplay in accordance with an embodiment of the invention. In theembodiment 120 illustrated in FIG. 17, a microdisplay 106 is configuredto emit unpolarized light 121 in a first wavelength band, which iscollimated and projected into a field of view by a collimator lens 107.The S-polarized emission from the microdisplay 106 can be coupled into atotal internal reflection path in a waveguide 101 by an S-diffractinginput grating 104 and extracted from the waveguide 101 by anS-diffracting output grating 105. P-polarized light from themicrodisplay 106 can be in-coupled and extracted using P-diffractinginput and output gratings 102,103 in a similar fashion. Dispersion canbe corrected for both S and P light provided that the input and outputgratings spatial frequencies are matched. The input and output gratings102,103 may be the deep SRGs or EBGs described above.

Although FIGS. 15-17 show specific waveguide display configurations,various configurations including modifications to those shown can beimplemented, the specific implementation of which can depend on thespecific requirements of the given application. Furthermore, suchdisplays can be manufactured using a number of different methods. Forexample, in many embodiments, the two grating layers are formed using aninkjet printing process.

In many embodiments, the waveguide operates in a monochrome band. Insome embodiments, the waveguide operates in the green band. In severalembodiments, waveguide layers operating in different spectral bands suchas red, green, and blue (RGB) can be stacked to provide a three-layerwaveguiding structure. In further embodiments, the layers are stackedwith air gaps between the waveguide layers. In various embodiments, thewaveguide layers operate in broader bands such as blue-green andgreen-red to provide two-waveguide layer solutions. In otherembodiments, the gratings are color multiplexed to reduce the number ofgrating layers. Various types of gratings can be implemented. In someembodiments, at least one grating in each layer is a switchable grating.

The invention can be applied using a variety of waveguidesarchitectures, including those disclosed in the literature. In manyembodiments, the waveguide can incorporate at least one of: anglemultiplexed gratings, color multiplexed gratings, fold gratings, dualinteraction gratings, rolled K-vector gratings, crossed fold gratings,tessellated gratings, chirped gratings, gratings with spatially varyingrefractive index modulation, gratings having spatially varying gratingthickness, gratings having spatially varying average refractive index,gratings with spatially varying refractive index modulation tensors, andgratings having spatially varying average refractive index tensors. Insome embodiments, the waveguide can incorporate at least one of: a halfwave plate, a quarter wave plate, an anti-reflection coating, a beamsplitting layer, an alignment layer, a photochromic back layer for glarereduction, louvre films for glare reduction In several embodiments, thewaveguide can support gratings providing separate optical paths fordifferent polarizations. In various embodiments, the waveguide cansupport gratings providing separate optical paths for different spectralbandwidths. In a number of embodiments, gratings for use in theinvention can be HPDLC gratings, switching gratings recorded in HPDLC(such switchable Bragg Gratings), Bragg gratings recorded in holographicphotopolymer, or surface relief gratings.

In many embodiments, the waveguide display can provide an image field ofview of at least 50° diagonal. In further embodiments, the waveguidedisplay can provide an image field of view of at least 70° diagonal. Insome embodiments, an OLED display can have a luminance greater than 4000nits and a resolution of 4k×4k pixels. In several embodiments, thewaveguide can have an optical efficiency greater than 10% such that agreater than 400 nit image luminance can be provided using an OLEDdisplay of luminance 4000 nits. Waveguide displays implementingP-diffracting gratings typically have a waveguide efficiency of 5%-6.2%.Providing S-diffracting gratings as discussed above can increase theefficiency of the waveguide by a factor of 2. In various embodiments, aneyebox of greater than 10 mm with an eye relief greater than 25 mm canbe provided. In many embodiments, the waveguide thickness can be between2.0-5.0 mm.

FIG. 18 conceptually illustrates a waveguide display in accordance withan embodiment of the invention in which at least one portion of at leastone of the waveguide optical surfaces is curved and the effect of thecurved surface portion on the guided beam wavefronts. As shown, theapparatus 130 includes a waveguide 131 supporting the curved surfaceportion 132. In the illustrative embodiment, the waveguide 131 supportsinput 102 and output 103 gratings with high diffraction efficiency forP-polarized light in a first wavelength band and input 104 and output105 gratings with high diffraction efficiency for S-polarized light inthe first wavelength band. The microdisplay 106, which displays arectangular array of pixels 133 emits unpolarized light 134 in the firstwavelength band, which is collimated and projected into a field of viewby a collimator lens 107. The P-polarized emission from the microdisplay106 can be coupled into a total internal reflection path into thewaveguide by the P-diffracting input grating 102 and extracted from thewaveguide by the P-diffracting output grating 103. The presence of anynon-planar surface in a waveguide can distort the waterfronts of theguided light such that the output light when viewed from the eyeboxexhibits defocus, geometric distortion, and other aberrations. Forexample, in FIG. 18, the light projected by the collimator lens 107 froma single pixel has planar wavefronts 135, which after propagatingthrough the waveguide 131 along the TIR path 136 forms non-paralleloutput rays 137-139 that are normal to the curved output wavefront 139A.On the other hand, a perfect planar waveguide would instead provideparallel beam expanded light. FIG. 19 conceptually illustrates a version140 of the waveguide in which the waveguide substrate 141 supports twooverlapping upper 142 and lower 143 curved surfaces.

FIG. 20 conceptually illustrates a waveguide display in accordance withan embodiment of the invention in which the aberrations introduced by acurved surface portion can be corrected by pre-distorting the pixelpattern of the OLED microdisplay. In the illustrative embodiment, thewaveguide apparatus 150 is similar to the one illustrated in FIG. 18. Asshown, the apparatus 150 includes a microdisplay 151 that supports apre-distorted pixel pattern 152. Unpolarized first wavelength light 153emitted by the microdisplay is focused by the lens 107, whichsubstantially collimates the beam entering the waveguide while formingwavefronts 154 that are pre-distorted by a small amount. Afterin-coupling and propagation 155 through the waveguide 131, thepredistorted wavefronts are focused by the curved surface 132 to formparallel output rays 156-158, which are normal to the planar outputwavefront 159.

FIG. 21 conceptually illustrates a waveguide display in accordance withan embodiment of the invention in which the aberrations introduced by acurved surface portion can be corrected by pre-distorting the pixelpattern of an OLED microdisplay formed on a curved substrate. The curvedmicrodisplay substrates can help to correct focus errors fieldcurvature, distortion, and other aberrations in association with thedistorted pixel pattern. In the illustrative embodiment, the waveguideapparatus 160 is similar to the one illustrated in FIG. 18. As shown,the curved substrate microdisplay 161 supports the pre-distorted pixelpattern 164. Unpolarized first wavelength light 163 emitted by themicrodisplay is focused by the lens 107 to form substantially collimatedguided beams with slightly pre-distorted wavefronts 164, which, afterin-coupling and propagation 165 through the waveguide 131, form paralleloutput rays 166-168 that are normal to the planar output wavefront 169.

Although FIGS. 18-21 show specific configurations of waveguides havingcurved surfaces, many other different configurations and modificationscan be implemented. For example, the techniques and underlying theoryillustrated in such embodiments can also be applied to waveguidessupporting eye prescription optical surfaces. In many embodiments,prescription waveguide substrates can be custom-manufactured usingsimilar processes to those used in the manufacture of eye prescriptionspectacles, with a standard baseline prescription being fine-tuned toindividual user requirements. In some embodiments, waveguide gratingscan be inkjet printed with a standard baseline prescription. In severalembodiments, the OLED display can be custom-printed with a pre-distortedpixel pattern formed. In various embodiments, the OLED display can beprinted onto a curved backplane substrate. In a number of embodiments,additional refractive or diffractive pre-compensation elements can besupported by the waveguide. In many embodiments, additional correctionfunctions can be encoded in at least one of the input and outputgratings. The input and output gratings may be the deep SRGs or EBGs orthe hybrid gratings described above and may be manufactured in themethods described in connection with FIGS. 1-5. The input and outputgratings may also have thicknesses described in connection with FIGS.6-8.

FIG. 22 is a flow chart conceptually illustrating a method forprojecting image light for view using a waveguide containingS-diffracting and P-diffracting gratings in accordance with anembodiment of the invention. As shown, the method 170 of forming animage is provided. Referring to the flow diagram, method 170 includesproviding (171) an OLED array emitting light in a first wavelengthrange, a collimation lens, and a waveguide supporting input and outputgratings with high diffraction efficiency for S-polarized light in afirst wavelength band and input and output gratings with highdiffraction efficiency for P-polarized light in the first wavelengthband. In some embodiments, the input and output gratings may be the deepSRGs, EBGs, or hybrid gratings discussed previously. Image light emittedby the OLED array can be collimated (172) using the collimation lens.S-polarized light can be coupled (173) into a total internal reflectionpath in the waveguide using the S-diffracting input grating. P-polarizedlight can be coupled (174) into a total internal reflection path in thewaveguide using the P-diffracting input grating. S-polarized light canbe beam expanded and extracted (175) from the waveguide for viewing.P-polarized light can be beam expanded and extracted (176) from thewaveguide for viewing.

FIG. 23 is a flow chart conceptually illustrating a method forprojecting image light for view using a waveguide supporting an opticalprescription surface and containing S-diffracting and P-diffractinggratings in accordance with an embodiment of the invention. As shown,the method 180 of forming an image is provided. Referring to the flowdiagram, method 180 includes providing (181) an OLED array with apredistorted pixel pattern emitting light in a first wavelength range, acollimation lens, and a waveguide supporting input and output gratingswith high diffraction efficiency for S-polarized light into a firstwavelength band and input and output gratings with high diffractionefficiency for P-polarized light in the first wavelength band andfurther providing (182) a prescription optical surface supported by thewaveguide. In some embodiments, the input and output gratings may be thedeep SRGs, EBGs, or hybrid gratings discussed previously. Image lightemitted by the OLED array can be collimated (183) using the collimationlens. S-polarized light can be coupled (184) into a total internalreflection path in the waveguide using the S-diffracting input grating.P-polarized light can be coupled (185) into a total internal reflectionpath in the waveguide using the P-diffracting input grating. Thepre-distorted wavefront can be reflected (186) at the prescriptionsurface. A planar wavefront can be formed (187) from the pre-distortedwavefront using the optical power of the prescription surface.S-polarized light can be beam expanded and extracted (188) from thewaveguide for viewing. P-polarized light can be beam expanded andextracted (189) from the waveguide for viewing.

Discussion of Embodiments Including Varied Pixel Geometries

The various apparatus discussed in this disclosure can be applied usingemissive displays with input pixel arrays of many different geometriesthat are limited only by geometrical constraints and the practicalissues in implementing the arrays. In many embodiments, the pixel arraycan include pixels that are aperiodic (non-repeating). In suchembodiments, the asymmetry in the geometry and the distribution of thepixels can be used to produce uniformity in the output illumination fromthe waveguide. The optimal pixel sizes and geometries can be determinedusing reverse vector raytracing from the eyebox though the output andinput gratings (and fold gratings, if used) onto the pixel array. Avariety of asymmetric pixel patterns can be used in the invention. Forexample, FIG. 24A conceptually illustrates a portion 230 of a pixelpattern comprising rectangular elements 230A-230F of differing size andaspect ratios for use in an emissive display panel in accordance with anembodiment of the invention. In some embodiments, the pixels array canbe based a non-repeating pattern based on a finite set of polygonal baseelements. For example, FIG. 24B conceptually illustrates a portion 240of a pixel pattern having Penrose tiles 240A-240J for use in an emissivedisplay panel in accordance with an embodiment of the invention. Thetiles can be based on the principles disclosed in U.S. Pat. No.4,133,152 by Penrose entitled “Set of tiles for covering a surface”.Patterns occurring in nature, of which honeycombs are well knownexamples, can also be used in many embodiments.

In many embodiments, the pixels can include arrays of identical regularpolygons. For example, FIG. 24C conceptually illustrates a portion 250of a pixel pattern having hexagonal elements in accordance with anembodiment of the invention. FIG. 24D conceptually illustrates a portion260 of a pixel pattern having square elements 250A-250C in accordancewith an embodiment of the invention. FIG. 24E conceptually illustrates aportion 270 of a pixel pattern having diamond-shaped elements 270A-270Din accordance with an embodiment of the invention. FIG. 24F conceptuallyillustrates a portion 280 of a pixel pattern having isosceles triangleelements 280A-280H in accordance with an embodiment of the invention.

In many embodiments, the pixels have vertically or horizontally biasedaspect ratios. FIG. 24G conceptually illustrates a portion 290 of apixel pattern having hexagonal elements 290A-290C of horizontally biasedaspect ratio. FIG. 24H conceptually illustrates a portion 300 of a pixelpattern having rectangular elements 300A-300D of horizontally biasedaspect ratio in accordance with an embodiment of the invention. FIG. 24Iconceptually illustrates a portion 310 of a pixel pattern having diamondshaped elements 310A-310D of horizontally biased aspect ratio inaccordance with an embodiment of the invention. FIG. 24J conceptuallyillustrates a portion 320 of a pixel pattern having triangular elements320A-320H of horizontally biased aspect ratio in accordance with anembodiment of the invention.

In many embodiments, OLEDs can be fabricated with cavity shapes andmulti-layer structures for shaping the spectral emission characteristicsof the OLED. In some embodiments microcavity OLEDs optimized to providenarrow spectral bandwidths can be used. In some embodiments, thespectral bandwidth can be less than 40 nm. In some embodiments, spectralbandwidth of 20 nm or less can be provided. In some embodiments, OLEDscan be made from materials that provide electroluminescent emission in arelatively narrow band centered near selected spectral regions whichcorrespond to one of the three primary colors. FIG. 25 conceptuallyillustrates a pixel pattern in which different pixels may have differentemission characteristics. In some embodiments, pixels may have differingspectral emission characteristics according to their position in thepixel array. In some embodiments, pixels may have differing angularemission characteristics according to their position in the pixel array.In some embodiments the pixels can have both spectral and angularemission characteristics that vary spatially across the pixel array. Thepixel pattern can be based on any of the patterns illustrated in FIGS.24A-24J. In many embodiments, pixels of different sizes and geometriescan be arranged to provide a spatial emission variation for controllinguniformity in the final image.

In many embodiments, OLEDs can have cavity structures designed fortransforming a given light distribution into a customized form. This istypically achieved by secondary optical elements, which can be bulky forwearable display application. Such designs also suffer from the problemthat they limit the final light source to a single permanent operationalmode, which can only be overcome by employing mechanically adjustableoptical elements. In some embodiments, OLEDs can enable real-timeregulation of a beam shape without relying on secondary optical elementsand without using any mechanical adjustment. In some embodiments, anOLED can be continuously tuned between forward and off axis principalemission directions while maintaining high quantum efficiency in anysetting as disclosed in an article by Fries (Fries F. et al, “Real-timebeam shaping without additional optical elements”, Light Science &Applications, 7(1), 18, (2018)).

An important OLED development, the “microcavity OLED”, may offerpotential for more controlled spectral bandwidths and emission angles insome embodiments. However, microcavity OLEDs are not yet ready forcommercial exploitation. In one embodiment (corresponding to a 2-microngrating with index modulation 0.1, an average index 1.65 and an incidentangle in the waveguide of 45 degrees) the diffraction efficiency of anSBG is greater than 75% over the OLED emission spectrum (between25%-of-peak points). Narrower bandwidth OLEDs using deeper cavitystructures will reduce bandwidths down 40 nm. and below.

Advantageously, the invention can use OLEDs optimized for use in theblue at 460 nm., which provides better blue contrast in daylight ARdisplay applications than the more commonly used 440 nm OLED as well asbetter reliability and lifetime.

In some embodiments, the emissive display can be an OLED full colorsilicon backplane microdisplay similar to one developed by KopinCorporation (Westborough, M A). The Kopin microdisplay provides an imagediagonal of 0.99 inch and a pixel density of 2490 pixels per inch. Themicrodisplay uses Kopin's patented Pantile™ magnifying lenses to enablea compact form factor.

Although the invention has been discussed in terms of embodiments usingOLED microdisplays as an input image source, in many other embodiments,the invention can be applied with any other type of emissivemicrodisplay technology. In some embodiment the emissive microdisplaycan be a micro LED. Micro-LEDs benefit from reduced power consumptionand can operate efficiently at higher brightness than that of an OLEDdisplay. However, microLEDs are inherently monochrome Phosphorstypically used for converting color in LEDs do not scale well to smallsize, leading to more complicated device architectures which aredifficult to scale down to microdisplay applications.

Although polymer grating structures have been discussed in terms of usewithin OLED array based waveguide displays, polymer grating structureshave advantageous synergetic applications with other classes ofdisplays. Examples of these displays include image generators using anon-emissive display technology such as LCoS and MEMS based displays.While LCoS based displays typically emit polarized light which may makethe polarization based advantages of polymer grating structures lessapplicable, polymer grating structures may provide an advantageousefficiency and manufacturing cost savings over conventional imprintedgratings. Further, polymer grating structures may be applicable invarious other non-display waveguide-based implementations such aswaveguide sensors and/or waveguide illumination devices.

Doctrine of Equivalents

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. A method for fabricating a deep surface reliefgrating (SRG), the method comprising: providing a mixture of monomer andliquid crystal; providing a substrate; coating a layer of the mixture ona surface of the substrate; applying holographic recording beams to thelayer to form a holographic polymer dispersed liquid crystal gratingcomprising alternating polymer rich regions and liquid crystal richregions; and removing at least a portion of the liquid crystal in theliquid crystal rich regions to form a polymer surface relief grating,wherein the liquid crystal rich regions comprise air gaps after removingat least a portion of the liquid crystal in the liquid crystal richregions.
 2. The method of claim 1, wherein the monomer comprisesacrylates, methacrylates, vinyls, isocynates, thiols,isocyanate-acrylate, and/or thioline.
 3. The method of claim 2, whereinthe mixture further comprises at least one of a photoinitiator, acoinitiator, or additional additives.
 4. The method of claim 2, whereinthe thiols comprise thiol-vinyl-acrylate.
 5. The method of claim 3,wherein the photoinitiator comprises photosensitive components.
 6. Themethod of claim 1, wherein providing a mixture of monomer and liquidcrystal comprises: mixing the monomer, liquid crystal, and at least oneof a photoinitiator, a coinitiator, multifunctional thiol, or additionaladditives; storing the mixture in a location absent of light at atemperature of 22° C. or less; adding additional monomer to the mixture;filtering the mixture through a filter of 0.6 μm or less; and storingthe filtered mixture in a location absent of light.
 7. The method ofclaim 1, further comprising refilling the liquid crystal rich regionswith a liquid crystal material.
 8. The method of claim 1, whereinremoving at least a portion of the liquid crystal comprises removingsubstantially all of the liquid crystal in the liquid crystal richregions.
 9. The method of claim 1, wherein removing at least a portionof the liquid crystal further comprises leaving at least a portion ofthe liquid crystal in the polymer rich regions.
 10. The method of claim1, wherein removing at least a portion of liquid crystal compriseswashing the holographic polymer dispersed liquid crystal grating with asolvent.
 11. The method of claim 5, wherein the photosensitivecomponents comprise dyes and/or radical generators.
 12. The method ofclaim 7, wherein the liquid crystal material has a different molecularstructure than the previously removed liquid crystal.
 13. The method ofclaim 1, wherein the substrate comprises a glass substrate or plasticsubstrate.
 14. The method of claim 1, wherein the substrate comprises atransparent substrate.
 15. The method of claim 1, further comprisingsandwiching the mixture between the substrate and another substrate withone or more spacers for maintaining internal dimensions.
 16. The methodof claim 15, further comprising applying a non-stick release layer onone surface of the other substrate.
 17. The method of claim 16, whereinthe non-stick release layer comprises a fluoropolymer.
 18. The method ofclaim 1, further comprising applying a protective layer over the deepSRG.
 19. The method of claim 18, wherein the protective layer comprisesan anti-reflective layer.
 20. The method of claim 18, wherein theprotective layer comprises silicate or silicon nitride.
 21. The methodof claim 18, wherein the protective layer comprises a parylene coating.22. The method of claim 1, further comprising creating a vacuum in theair gaps or filling the air gaps with an inert gas.
 23. The method ofclaim 1, wherein removing at least a portion of liquid crystal compriseswashing the holographic polymer dispersed liquid crystal grating with asolvent.
 24. The method of claim 23, wherein washing the holographicpolymer dispersed liquid crystal grating comprises immersing theholographic polymer dispersed liquid crystal grating in the solvent. 25.The method of claim 1, further comprising curing the holographic polymerdispersed liquid crystal grating.
 26. The method of claim 1, whereincuring the holographic polymer dispersed liquid crystal gratingcomprises exposing the holographic polymer dispersed liquid crystalgrating to a low intensity white light for a period of about an hour.27. The method of claim 1, wherein the polymer surface relief gratingextends all the way to contact the substrate.
 28. The method of claim27, wherein the polymer surface relief grating directly contacts thesubstrate.
 29. The method of claim 28, wherein there is no bias layerbetween the polymer surface relief grating and the substrate.
 30. Amethod for fabricating a deep surface relief grating (SRG), the methodcomprising: mixing a mixture comprising a monomer, a liquid crystal, andat least one of a photoinitiator, a coinitiator, multifunctional thiol,or additional additives; storing the mixture in a location absent oflight at a temperature of 22° C. or less; adding additional monomer tothe mixture; filtering the mixture through a filter of 0.6 μm or less;storing the filtered mixture in a location absent of light; providing asubstrate; coating a layer of the mixture on a surface of the substrate;applying holographic recording beams to the layer to form a holographicpolymer dispersed liquid crystal grating comprising alternating polymerrich regions and liquid crystal rich regions; and removing at least aportion of the liquid crystal in the liquid crystal rich regions to forma polymer surface relief grating.